Continuations or suspend and resume in the Servlet 3.0 API for communication with the client?

Question

We are in the process of migrating a Swing two tier application to a web application (tomcat, Spring MVC, extJS if it makes any difference).

We found code like this in the application (simplified Javaesc pseudo code).

class DoSomethingComplicatedAction extends Action{
    public performAction(..){
        // do lots of stuff here
        // decend about 40 steps in the call stack
        answer = JOptionPane.showConfirmDialog()
        if (answer == something){
            // do something convoluted here
        } else {
            // do something even more convoluted here
        }
}

i.e. in the middle of some processing, some interaction with the user is started. Obviously I can't do that directly with a servlet.

One thing that could handle this rather nicely are continuations. So I checked if I can use those and were surprised that there are actually libraries that enable this kind of stuff: Continuations in Java

When checking the libraries mentioned in that question and its answers I ran across this statement

Continuations will be replaced by standard Servlet-3.0 suspendable requests once the specification is finalized. Early releases of Jetty-7 are now available that implement the proposed standard suspend/resume API

But I couldn't find an example how to do stuff like the above with the Servlet 3.0 API

So the questions are:

1. Can the above be done using the Servlet-3.0 API without completely refactoring the code sketched above into two or more separate actions

2. If the answer to the above is yes: How? Are there somewhat complete examples for this or a similar use case available?

3. Should I use Continuations or the Servlet API? Or if this can't be answered directly on what conditions does this decision depend on?

Answer 1

The continuation api proposed by jetty was a leading workaround to overcome the limitations of the servlet api at the time. (one request, one thread)

Now, you should be pretty much only use the Servlet 3.0 only.

Note that in your case, you may not even need to do anything special, because Actions are re-active, and you can go the usual ajax way.

I am adding a few relevant links:

1. Ajax, Reverse Ajax
2. Async vs sync servlets
3. Jetty 8.0 and continuations

Answer 2

I'd say that a more reasonable approach to porting a Swing application to a web application would be to use GWT. This would allow you to not only do what you're asking, but it would also provide a much faster UI, since there would be more processing happening on the client (i.e. no roundtrips to the server).

Also, you'd be able to reuse a lot of the existing client code, since GWT is pretty much standard Java (with some restrictions).